X-rays are currently used to scan luggage in security systems. Typically, a dual-energy X-ray system is used to distinguish organic, inorganic and metal materials. Most explosives, biological agents that can be used for bioterrorism, and drugs (controlled substances) fall within the broad organic materials category. Since the X-ray system does not specifically identify chemical compositions, detection of organic materials can result in false positives and the need for further examination.
The use of nuclear quadrupole resonance (NQR) as a means of detecting explosives, drugs and other contraband has been recognized for some time; see, e.g., T. Hirshfield et al, J. Molec. Struct. 58, 63 (1980); A. N. Garroway et al, Proc. SPIE 2092, 318 (1993); and A. N. Garroway et al, IEEE Trans. on Geoscience and Remote Sensing 39, 1108 (2001). NQR provides some distinct advantages over other detection methods. NQR requires no external magnet such as required by nuclear magnetic resonance, and NQR is sensitive to the compounds of interest, i.e. there is a specificity of the NQR frequencies. Since NQR provides this specificity it can identify particular compositions, e.g. specific explosives, biological agents that can be used for bioterrorism, and drugs.
A NQR detection system can have one or more dual-purpose coils that serve as both excitation and receive coils, or it can have separate coils that only excite and only receive. An excitation, i.e. transmit, coil of a NQR detection system provides a radio frequency (RF) magnetic field that excites the quadrupole nuclei in the sample and results in their producing their characteristic resonance signals that the receive coil, i.e. sensor, detects.
It can be especially advantageous to use a sensor made of a high temperature superconductor (HTS) rather than copper since the HTS self-resonant coil has a quality factor Q of the order of 103-106. The NQR signals have low intensity and short duration. In view of the low intensity NQR signal, it is important to have a signal-to-noise ratio (S/N) as large as possible. The signal-to-noise ratio is proportional to the square root of Q so that the use of a HTS self-resonant coil as a sensor results in an increase in S/N by a factor of 10-100 over that of a copper coil. Therefore, the use of a high temperature superconductor coil with a large Q as the sensor provides a distinct advantage over the use of an ordinary conductor coil.
A combined nuclear quadrupole resonance and X-ray detection system provides the existing detection capabilities of the X-ray system with the specific compound detection capabilities of the NQR system. Particular contraband can be unequivocally detected by NQR. This eliminates the uncertainty connected with false positives of the X-ray system. The detection of sheet explosives is one of the capabilities of the NQR system.
The metal shielding of contraband such as explosives, biological agents that can be used for bioterrorism, and drugs can present a problem to both X-ray and NQR detection systems. A thick metal shield will be detected by an X-ray system and prompt further examination. However, a thin metal shield will be essentially transparent to X-rays. For a NQR system, a metal shield as thin as a 25μ thick aluminum foil will prevent detection by identification of a particular NQR frequency that is characteristic of a particular target substance. Sheet explosive is one type of contraband for which thin metal shielding might be used.
An object of the present invention is to provide a combined nuclear quadrupole resonance and X-ray detection system with a metal shield alarm that will signal the existence of metal not detected by X-ray and of sufficient area to shield contraband, e.g. sheet explosives, from NQR detection.